x86: fix MTRR initialization if EARLY_AP_STARTUP is used

MTRR handlers are set in {amd64/i686}_mem_drvinit, which is called at
SI_SUB_DRIVERS, and that's too late when EARLY_AP_STARTUP is set because APs
have already started at this point. {amd64/i686}_mrinit is also called too late
for the BSP, since that happens when the memory device is attached, also after
APs have already started.

Move the position to SI_SUB_CPU, and also initialize the state for the BSP, so
that the APs can correctly get to the same state as the BSP.

Sponsored by: Citrix Systems R&D
MFC after: 1 week
Reviewed by: jhb, kib
Differential Revision: https://reviews.freebsd.org/D9630